The invention concerns a pump for cryogenic fluids. Such pumps are used when pressure resistant commercial steel cylinders are to be filled with gas, such as for example nitrogen at high pressure.
Nitrogen is produced from liquified air under low temperature. It is stored in an insulated storage tank at a temperature of approximately -196.degree. C. and under low vapor pressure of approximately 2 bar.
It is put on the market however in high pressure steel bottles in which the nitrogen is in gaseous condition at room temperature and under a pressure of approximately 200 bar.
The cryogenic pump has the task of pumping the liquid nitrogen from the storage tank and raising its pressure to about 200 bar, so that after evaporation it can be placed into the high pressure steel bottles.
In the pumping of cryogenic fluids, special difficulties are encountered due to the fact that the fluid converts from the liquid to the gaseous phase by a decrease in pressure as well as by a rise of temperature. At the beginning the pump is at atmospheric pressure and room temperature and must be cooled down to approximately the temperature of the cryogenic fluid. During operation, the conditions must be brought to over and above the conditions of the vapor pressure curve of the cryogenic fluid being pumped. This is because on the suction stroke of the pump the pressure decreases, giving rise to gas formation.
Known measures to avoid such problems include the following:
1. Conveying the fluid from the large storage tank in which vapor pressure conditions prevail into a closed intermediate container which is heat insulated to the best possible extent, and lowering the temperature below the temperature of the vapor pressure.
2. Increasing the pressure in the intermediate container above the vapor pressure.
To implement the last mentioned measure, a pump is disclosed in Swiss Patent Specification No. CH-PS 615,982 which has a stepped or differential piston, a hollow piston rod and valves disposed in the piston. The high degree of compression gives rise to corresponding gas forming currents which must be conveyed by means of an elaborate valve system out of the low pressure part. Pumps of this type are complicate and expensive to manufacture.
It is the object of the present invention to create a pump as recited in claim 1, which pump operates according to this principle, but which is structurally less complex and more cost efficient to manufacture and which causes a lesser degree of gas formation during the precompression phase.
This is accomplished according to the invention by providing a cryogenic pump which incorporates the specific features recited in claim 1.